Anastigmatic deflection yoke



July 31, 1951 K. SCHLESINGER 2,562,395

ANASTIGMATIC DEFLECTION YOKE Filed Feb. 23, 1949 INVENTOR. Kurt Schlesinger Patented July 31, 1951 ANASTIGMATIC DEFLEGTION YOKE Kurt Schlesinger, Maywood, Ill., assignor to Motorola, Inc., Chicago, 111., a corporation of Illinois Application February 23, 1949, Serial No. 77,855

4 Claims.

This invention relates generally to deflection systems and more particularly to an electromagnetic deflection yoke which produces uniform magnetic field extending perpendicular to each other.

In television systems and other apparatus in which cathode ray tubes are used to provide visual indications, yokes are provided about the necks of the tubes for deflecting the cathode ray beam thereof for producing such indications. These yokes may include separate coils for producing fields extending in two directions perpendicular to each other a the fields used for horizontal and vertical deflection of the beam in television equipment. The provision of a simple coil structhe neck of the tube and with an effective gap ture about the neck of the tube, however, has not resulted in completely uniform field through the tube with the result that geometric distortion of the visual indication is produced. Such non-uniform fields also have the effect of defocusing the cathode ray beam so that certain portions of the indication may be blurred. Although it is known that a uniform field may be produced through a cylindrical space by providing a sinusoidally distributed winding about the space, such a winding is very difficult to construct and would result in 7 a very expensive yokestructure. In my prior application, Serial No. 2,493, filed January 15, 1948, subject: Electromagnetic Deflection Yoke, there is described a yoke structure which provides undistorted or anastigmatic deflection of the beam of a cathode ray tube. The present application is directed to a different structure of simpler form for producing the same general results.

It is, therefore, an object of the present invention to provide an improved yoke structure for between the adjacent sides of each pair of coils so that a uniform field is provided by each pair of coils.

A still further feature of this invention i the provision of a yoke for a cathode ray tube having coils for producing deflection in directions perpendicular to each other and transverse to the neck of the tube, in which the coils have sides extending along the tube neck in the space extending from an angle of about 10 degrees toan angle of about degrees from the radial plane through the neck which is perpendicular to the field produced by each pair of coils.

Further objects, features and advantages will be apparent from a consideration of the following description when taken in connection with the accompanying drawings in which:

Fig. 1 illustrates a cathode ray tube with the yoke positioned thereon;

v Fig. 2 is a cross-sectional view of the yoke in accordance with the invention;

Figs. 3 and 4 are cross-sectional views illustrating the field produced by yokes of 'differen constructions; I,

r Fig. 5 is a set of curves illustrating factors required. for eliminating distortion in yoke structures; and

t Fig. 6 shows another embodiment of the inven-' ion.

In practicing the invention there is provided a deflection yoke including a pair of coils for producing horizontal deflection and a second pair of coils for producing vertical deflection. .These coils have elongated sides positioned to define a cylindrical space so that the yoke may be posi-r tioned about the neck of a cathode ray tube. The sides of the various coils are interspersed so that the coils are positioned in a single layer about the cylindrical space. The coils for producing deflection in each direction have the sides thereof A feature of this invention is the provision of U a deflection yoke structure in which the coils define a cylindrical space and the turns distribution thereof is such that the field produced in the space thereby is uniform.

A further feature of this invention is the provision of a yoke for a cathode ray tube including individual pairs of coils for producing horizontal and vertical fields having sides extending along Y of the coils to these values.

positioned on opposite sides of the radial plane through the space which is parallel to the direction of deflection. To provide a uniform field, only the portions of the coils positioned in the arcuate' space extending from an angle of the order of 10 degrees to an angle of the order of 45 degrees from the radial plane are effective in producing the fields. This may be accomplished by shielding a portion of the coils or limiting the extent It will be apparent that this latter construction will produce a yoke having four slots or open parts uniformly spaced about the circumference thereof.

Referring now more particularly to the draw-j ings, in Fig. 1 there is illustrated a cathode ray tube ill having a yoke ll positioned on the neck l2 of the tube. In Fig. 2, the construction of the yoke in accordance with th invention is shown more in detail. The yoke of Fig. 2 includes coils l5 and [6 for producing horizontal deflection and coils l1 and [8 for producing vertical deflection. The ends of the coils are brought around the neck of the tube in a well known manner and as illustrated generally in Fig. 1. The fields for producing the deflections are perpendicular to the direction of deflection so that the coils l5 and I6 produce a vertical field, and the coils I! and I8 produce a horizontal field. It is to be noted that the coils I5 and iii are spaced from the horizontal diameter by an angle of the order of 10 degrees and extend about the tube to an angle of the order of 45 degrees. The coils must necessarily be included within the angle of 45 degrees so that the proper space is provided for the coils for producing vertical deflection. The coils may be provided on a cylindrical form l3, having portions l4 thereon in the spaces between the coils. It is to be pointed out, however, that other types of forms may be used.

In standard deflection yokes two coils are provided for producing deflection in each direction (horizontal and vertical) with the coils having sides extendingthrough an arc of 45 degrees. The eight sides of the four coils therefor fill up the entire space about the neck of the tube. This is illustrated in Fig. 3 in which the vertical coils 22 and 23 and the horizontal coils and 2i completely encompass the cylindrical form 24. It has been found in practice and it can be shown mathematically that such a coil distribution produces a field in which the flux lines are curved as indicated at 25. It is obvious that such a coil or turn distribution does not produce a uniform field and will cause distortion in the picture reproduced by'the tube. The distortion produced by such a distribution is generally called barrel distortion. Such a field distribution also causes defocusing of the beam as will be more fully explained. v

In order to determine the configuration of a. coil which produces a'uniform field within such a cylindrical space, wemust consider the relationship between the fiux in the space and the potential distribution about the surface of the cylindrical space. It is generally true that the fields produced by magnetic coils have an electrostatic equivalent in the application of certain potentials about the surfac of the cylindrical space. We may designate this equivalent surface potential at any point on the surface P, and then the potential at any point inside the cylindrical space may be represented by the following polar equation:

=a1r sin a+CL3T sin 3a+a5 +av where is the potential at any point inside having coordinates r and a, where r is the radius and a represents the angle, in and a2 are constants which are determined by the equivalent surface potential P, with as depending upon the third harmonic of the equivalent surface potential. The above equation is simplified by considering only the third order terms. This approximation is sufiiciently accurate for television applications. Higher order corrections ar more important if the beam being deflected is of large cross-section but are not material when the beam fills only about one percent of the neck of the tube. It is apparent from this. equat on that in order for the potential distribution to be linear, the term including us must be zero.

In order to evaluate the above equations, the in and as terms must be considered more in detail. The following indicates the nature of these values:

1 4 4 a P Sll'l ada and P is the boundary potential and in general is directly related to the ampere turns density on the surface. For the present we can write P=fnda, where nis the turns density or the number of turns per unit of angle. It is apparent from the above, therefore, that m indicates the gradient of the potential and when considered in a yoke is a measure of the sensitivity produced by the yoke. as indicates the distortion of the fields.

When considering a field produced by a coil such as coil I5 of Fig. 2, the as term of the above equation can be made to equal zero in the manner illustrated by the curves of Fig. 5. Curve A of Fig. 5 is a curve representing the values of sin 3:: from zero to degrees. Curve B shows n the winding distribution, with the point 30 being fixed at an upper limit of about 45 degrees and the point ill to be positioned at the angle providing a minimum of distortion. Curve C is the integral of the winding distribution and therefore represents P. Curve D show the prodnot of curves A and C and this curve therefore represents P sin 30.. In order to make the integral of this quantity zero from 0 to 90, the area under the curve must be equal to zero, or in other words, the positive and negative areas must be equal. By making the positive and negative portions of the area under the curve equal, as therefore becomes zero and the distortion is eliminated. It can be shown both geometrically, and by solving the above equations that when the point 3! is at 15 the areas are exactly equal.

It is helpful in considering the effect of the field produced by the surface potentials to consider also the equation in rectangular coordinates. The following equation is exactly the same as the polar equation stated above:

The terms in and as are the same quantities defined above with in indicating the sensitivity and as the distortion. Actually, the quantity 3a3X represents the geometric distortion and the term (Z3Y represents defocusing. It is apparent from this equation that when as is equal to zero, both the geometric distortion and the defocuslng will be corrected simultaneously.

In yokes constructed, it has been found that the angle of the slots (a, Fig. 2) should be somewhat less than the angle computed above. Tests indicate that the angle should be in the region from 8 to 15 degrees. This discrepancy is. caused in part by the simplifying assumptions, made in the mathematical analysis and in part by mechanical limitations. The wires from which the coils are made have material size and are somewhat stiff. The coils therefore will not fit along the form in the manner indicated. The ends of the coils are bent up as indicated in git th resul that the slots a e w der at the ends. These various factors require that the angle of the slots be reduced and yokes in which the angle is of the order of degrees provide the best correction.

Referring again to Fig. 3, the curved flux lines 24 produced in this structure will cause horizontal lines 26 atthe top and bottom of the oathode ray tube screen which are bowed out from the center. This is what is known as barrel distortion. The dots 28 about the edge of the screen will be defocused and will appear elongated radially. The dots at the center of the screen such as 2'! will not be substantially defocused and will appear substantially round. It has been found by experiment that the angle by which the coils of each assembly are spaced from the radial plane separating the coils is relatively critical. If the coils are spaced by an angle greater than degrees, as illustrated at a in Fig. l, over compensation occurs. In this figure the coils 453 and 4! produce horizontal deflection and the coils 42 43 produce vertical deflection. It is noted that the flux lines 44 are bowed out from the center resulting in the horizontal lines 45 of the indication being bowed toward the center. This produces geometric distortion which is generally called pillow distortion. Such a field produces defocusing which elongates dots in a tangential di rection as indicated by the dots 4B, or in other words, tangential defocusing. The dots 41 at the center of the picture are of course not substantially affected and remain in focus.

Distribution of the coils as shown in Fig. 2 produces third order correction with substantially uniform fields as indicated by the linear flux lines 19. Such fields eliminate both geometric distortion and defocusing so that the picture is not only sharp in the center but at all points on the screen. This is particularly important where large angle deflection is used and a large visual indication is provided.

In Fig. 6 there is illustrated a modification of the invention in which the coils are positioned in the manner illustrated in Fig. 3. However, the efl'ect similar to that produced by slots or spaces between the coils is provided by the use of magnetic members which blank out the effect of portions of the coils. In Fig. 6, horizontal deflection is produced by coils 5B and 5| and vertical deflection is provided by coils 52 and 53. Members 54 and 55 are associated with the horizontal deflecting coils 50 and 5| to pro vide a shield which blanks out or shields portions of the coils adjacent the horizontal diameter of the tube. This has the same effect on distortion as the provision of slots which provide dead angles. The magnetic members will have an effect on the intensity of the fields also, but this can be compensated for by the energy applied to the deflecting coils.

From the above it is seen that a yoke for a cathode ray tube has been provided which produces uniform fields extending perpendicular with respect to each other. Such fields provide deflection which is correct geometrically and the fields do not cause defocusing of the beam such as that produced by the non-uniform fields previously used. The structure is simple requiring only two coils for deflection in each direction as is used in standard yokes. Although the sensitivity is reduced slightly by elimination of certain portions of the coils which form the slots, the decrease in sensitivity is not large and structures as disclosed have been found to be entirely practical.

Although certain embodiments of the invention have been described which are illustrative thereof, it is obvious that various changes and modifications can be made therein without departingfrom the intended scope of the invention as defined in the appended claims.

I claim:

1. A deflection yoke for a cathode ray tube comprising a pair of coil assemblies each of which produces a substantially uniform magnetic field extending transversely through the cylindrical neck of said tube with one of said coil assemblies producing a vertical field for horizontal deflection of the beam of said tube and the other one of said coil assemblies producing a horizontal field for vertical deflection of said beam, each of said assemblies consisting of a pair of coils having elongated sides extending longitudinally alongthe outer surface of said neck of said tube. with the sides of all of said coils being of sub stantially uniform radial depth and being positioned in a single layer about said tube neck, said coils of said one coil assembly being positioned on the opposite sides of the horizontal radial plane of said neck and the coils of said other coil assembly being positioned on the opposite sides of the vertical radial plane of said neck, each side of said coils being positioned in the space around the outer surface of said neck extending from an angle of the order of 10 degrees to an angle of approximately 45 degrees from the adjacent one of said radial plane.

2. A deflection yoke for a cathode ray tube including a cylindrical coil form, a pair of coil assemblies on said form each of which produces a substantially uniform magnetic field through said form with the fields produced by said pair of assemblies extending substantially at right angles to each other, said form including outwardly projecting portions extending through an angle of the order of 8 on each side of the radial planes through said form in the directions of said fields, each of said coil assemblies consisting of a pair of elongated coils having sides extending along the outer surface of said form in a direction parallel to the axis of said form, said sides of all said coils being of substantially uniform radial depth and being positioned in a single layer about said coil form, said sides of said coils of each assembly extending substantially from said outwardly projecting form portions around the outer surface of said form to an angle of approximately 45 on each side of said radial planes.

3. A deflection yoke for a cathode ray tube comprising a pair of coil assemblies each of which produces a substantially uniform magnetic field extending transversely through the cylindrical neck of said tube with one of said coil assemblies producing a vertical field for horizontal deflection of the beam of said tube and the other one of said coil assemblies producing a horizontal field for vertical deflection of said beam, each of said assemblies consisting of a pair of coils having elongated sides extending longitudinally of said neck of said tube, with the sides of all said coils being of substantially uniform radial depth and being positioned in a single layer about said tube neck, said coils of said one coil assembly being positioned on the opposite sides of the horizontal radial plane of said neck and said coils of said other coil assembly being positioned on the opposite sides of the vertical radial plane of said neck, said sides of said coils extending produces a substantially uniform magnetic field 10 extending transversely through the cylindrical form, with said fields produced by said assemblies being positioned at right angles with respect to each other, each of said assemblies consisting of a pair of elongated coils having sides extending in an axial direction along the surface of said form on each side of the radial plane through said fOlill which is perpendicular to the magnetic field produced by the particular coil assembly, said sides of all said coils having substantially uniform radial depth and being positioned in a single layer about said form, each of said coil sides extending around the circumference of said form from said radial planes through an angle of approximately 45 degrees, and arcuate magnetic shielding members secured to the inside of said form and extending on each side of said radial planes through an angle of the order of 8 degrees for changing the effect of said coils adjacent said radial planes.

KURT SCI-ILESINGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,172,733 Federmann et a1. Sept. 12, 1933 2,108,523 Bowman-Manifold Feb. 15, 1938 2,167,379 Tolson July 25, 1939 2,172,733 Federmann et al. Sept. 12, 1939 2,195,470 Roosenstein et al. Apr. 2, 1940 2,207,777 Blain July 16, 1940 2,217,409 Hepp Oct. 8, 1940 2,229,977 Kenyon Jan. 28, 1941 2,230,111 Gunther Jan. 28, 1941 2,236,498 Blain Apr. 1, 1941 2,240,606 Bobb May 6, 1941 2,333,806 Maurer Nov. 9, 1943 2,395,736 Grundmann Feb. 26, 1946 2,128,947 Torsch Oct. 14, 1947 2,461,239 Obert Feb. 8, 1949 

